FIGS. 1A through 1D illustrate an example of a standard ball valve for controlling fluid flow. A ball 100 with a fluid passage 102 therethrough is positioned within a valve body 104 between valve end caps 106. Each end cap 106 is provided with a fluid passage 108. A valve seat 110 is positioned between each end cap 106 and ball 100, each valve seat having a ball-engaging surface 112 and a passage 114 therethrough. Each valve seat 110 forms a substantially fluid-tight seal through engagement of ball-engaging surface 112 and the surface of ball 100. In the fully open position, ball 100 is rotated so that passage 102 forms (substantially collinearly) with passages 108 and 114 a continuous fluid passage though the ball valve. In the closed position, ball 100 is rotated so that passage 102 does not communicate with passages 108 and 114. In addition to closed and fully open, intermediate rotational positions of ball 100 correspond to partially closed (equivalently, partially opened) valve positions, enabling the ball valve to be used for flow control and/or throttling.
Each valve seat 110 is axially constrained by engagement of surface 112 by the ball 100. In both the fully opened and the closed valve positions, a valve seat 110 is axially constrained by engagement of surface 112 with the surface of ball 100 along the entire circumference of valve seat 110. In a partially opened position, engagement of surface 112 with the surface of the ball 100 extends around only a portion of the circumference of the valve seat 110. It is possible for the non-engaged portion of the circumference of valve seat 110 to be axially displaced by hydrodynamic forces arising from fluid flow through the ball valve. This problem is exacerbated under high-flow-rate and/or high temperature conditions. Such axial displacement may be of particular concern on the input side of the ball valve, and may degrade the fluid-tight characteristics of the ball valve or even lead to damage to the valve seat 110. A valve seat 110 thus displaced may become trapped between an edge of passage 102 and valve body 104 or end cap 106 when the valve is closed and thereby suffer damage.
It is therefore desirable to provide apparatus and methods for retaining a ball valve seat in its properly-engaged position within a ball valve. It is therefore desirable to provide apparatus and methods for retaining a ball valve seat in its properly-engaged position within a ball valve when the ball valve is in a partially-opened position. It is therefore desirable to provide apparatus and methods for retaining a ball valve seat in its properly-engaged position within a ball valve wherein the entire circumference of the valve seat is axially constrained. It is therefore desirable to provide apparatus and methods for retaining a ball valve seat in its properly-engaged position within a ball valve wherein the entire circumference of the valve seat is axially constrained when the ball valve is in a partially-opened position.